When running, each foot lands on the lateral side of the heel and then the center of the load moves to the medial side of the front foot portion. In this process, the heel pronates. Suppressing excessive pronation serves to prevent leg injuries due to running.
U.S. Pat. No. 6,199,302 discloses a shoe sole in which the medial side of the middle foot portion and the rear foot portion of the midsole is formed by a harder material than other portions of the midsole.
[First Patent Document] U.S. Pat. No. 6,199,302 (FIG. 1)